Retroreflective sheeting has the ability to redirect light incident upon a major surface of the sheeting toward its originating source. This unique ability has led to the wide-spread use of retroreflective sheeting in a wide variety of conspicuity enhancement applications relating to traffic and personal safety marking. Typical examples of uses of retroreflective sheeting include the placement of such sheetings on road signs, traffic cones and barricades to enhance their conspicuity, particularly under poor lighting conditions, such as night-time driving conditions or in conditions of inclement weather. These uses typically allow the sheeting to be adhered to relatively flat and rigid surfaces, thereby allowing the sheeting to be relatively inflexible. Additionally, sign applications are characterized by relatively predictable, standardized viewing geometries.
There are essentially two types of retroreflective sheeting, beaded sheeting and cube corner sheeting. Beaded sheeting employs a multitude of independent glass or ceramic microspheres to retroreflect incident light. From the optics perspective, beaded sheeting typically exhibits favorable rotational symmetry and entrance angularity performance because of the symmetrical nature of the beads. Additionally, beaded sheeting typically exhibits relatively good flexibility because the beads are independent from one another. However, beaded sheeting tends to exhibit relatively low brightness when compared to cube corner sheeting.
Cube corner retroreflective sheeting typically employs an array of rigid, interconnected cube corner elements to retroreflect light incident on a major surface of the sheeting. The basic cube corner element is a generally tetrahedral structure having three mutually substantially perpendicular lateral faces which intersect at a single reference point, or apex, and a base triangle opposite the apex. The symmetry axis, or optical axis, of the cube corner element is the axis which extends through the cube apex and trisects the internal space of the cube corner element. In conventional cube corner elements which have an equilateral base triangle, the optical axis of the cube corner element is perpendicular to the plane which contains the base triangle.
In operation, light incident on the base of the cube corner element is reflected from each of the three lateral faces of the element and is redirected toward the light source. Retroreflective sheeting generally incorporates a structured surface including at least one array of cube corner reflective elements to enhance the visibility of an object. When compared with beaded sheeting, cube corner retroreflective sheeting exhibits relatively greater brightness in response to light incident at relatively low entrance angles, for example, near normal light. However, cube corner retroreflective sheeting also exhibits relatively poor rotational symmetry performance at high entrance angles. In addition, cube corner retroreflective sheeting is typically stiffer than beaded sheeting because the cube corner elements are often all interconnected.
The optics of cube corner retroreflective sheetings can be designed to exhibit optimal performance at a specific orientation. This can be accomplished by forming the cube corner elements of the retroreflective sheeting such that their optical axes are canted relative to an axis perpendicular to the base plane of the sheeting. U.S. Pat. No. 4,588,258, issued to Hoopman on May 13, 1986 (""258 Patent), the teachings of which are incorporated by reference herein, discloses retroreflective sheeting which employs optics having canted cube corner elements which form opposing matched pairs. The sheeting disclosed in the ""258 Patent exhibits a primary plane of improved retroreflective performance at high entrance angles, identified as the x-plane in the ""258 Patent, and a secondary plane of improved retroreflective performance at high entrance angles, identified as the y-plane in the ""258 Patent.
In another patent, U.S. Pat. No. 2,380,447, issued to Jungersen on Jul. 31, 1945 (""447 Patent), the teachings of which are incorporated by reference herein, discloses in FIG. 15 of the ""447 Patent the optical axis of prism pairs tilting away from the common edge.
In another patent, U.S. Pat. No. 5,171,624, issued on Dec. 15, 1992 to Walter, the teachings of which are incorporated herein in its entirety by reference, discloses microprism reflective sheeting in which prism pairs are tilted with respect to one another at an angle of in the range of between about three and ten degrees, prism size of 0.15-0.64 mm (0.006-0.025 inches) (space between apices) and wherein at least one prism side surface is arcuate. It has been found that the arcuate shaped prism surfaces have been found to be extremely difficult to manufacture.
However, a need still exists for a relatively easy to manufacture retroreflective structure that provides a more uniform distribution of light.
The present invention is directed to a retroreflective structure having cube corner retroreflective elements. The structure includes a retroreflective sheeting having an array of transparent prisms formed into pairs of prisms. Each prism includes a base aperture and three intersecting lateral faces which meet at an apex. Each of the lateral faces includes a base edge which forms a portion of the perimeter of the base aperture. The base edge of each lateral face intersects the base edge of a contiguous lateral face to form a base point, wherein a first face of at least one prism in the array includes a first face first planar surface and a first face second planar surface. The first face first planar surface and the first face second planar surface are contiguous along an edge having a first end point and a second end point, wherein the apex, the first end point, and a first base point are coplanar and form a continuous edge from the first base point to the apex.
In a preferred embodiment, a second face of the prism having the first face first planar surface and first face second planar surface includes a second face first planar surface and a second face second planar surface. In another preferred embodiment, a third face of the prism having the first face first planar surface and first face second planar surface includes a third face first planar surface and a third face second planar surface. The planar surfaces can form a concave or convex shape as viewed from the exterior of the prism.
In one embodiment, the retroreflective structure includes a prism having a length along at least one base edge between about 0.002 and 0.05 inches (0.0508 and 1.27 millimeters). Preferably, there is a negative tilt between prism pairs. The array of prisms can be canted between about negative one and negative fifteen degrees. In an alternative embodiment, the prisms are positively canted between about one and fifteen degrees. In a preferred embodiment, the retroreflective structure includes a metalized layer on the facet side of the retroreflective elements for specular reflection.
In accordance with further aspects, a mold for casting retroreflective prisms is provided which includes a plurality of grooves in a body of mold material. The grooves intersect at an angle to form an array of prisms formed into pairs of prisms. Each prism includes a base aperture and three intersecting lateral faces which meet at an vertex. Each of the lateral faces has a base edge which forms a portion of the perimeter of the base aperture. The base edge of each lateral face intersects the base edge of a contiguous lateral face to form a base point. A first face of at least one prism in the array includes a first face first planar surface and a first face second planar surface. The first face first planar surface and the first face second planar surface are contiguous long an edge having a first end point and a second end point, wherein the vertex, the first end point, and a first base point are coplanar and form a continuous edge from the first base point to the vertex.
A second face of the prism having the first face first planar surface and first face second planar surface can include a second face first planar surface and a second face second planar surface. A third face of the prism having the first face first planar surface and first face second planar surface can include a third face first planar surface and a third face second planar surface.
In accordance with further aspects, a method of forming retroreflective sheeting is provided which includes the following steps: Forming a first mold by forming a plurality of grooves in a body of mold material, the grooves intersecting at an angle to form an array of prisms formed into pairs of prisms. Each prism includes a base aperture and three intersecting lateral faces which meet at an apex. Each of the lateral faces include a base edge which forms a portion of the perimeter of the base aperture. The base edge of each lateral face intersects the base edge of a contiguous lateral face to form a base point, wherein a first face of at least one prism in the array includes a first face first planar surface and a first face second planar surface. The first face first planar surface and the first face second planar surface are contiguous along an edge having a first end point and a second end point, wherein the apex, the first end point, and a first base point are coplanar and form a continuous edge from the first base point to the apex.
A second face of the prism having the first face first planar surface and first face second planar surface can include a second face first planar surface and a second face second planar surface. A third face of the prism having the first face first planar surface and first face second planar surface can include a third face first planar surface and a third face second planar surface.
The method further includes the step of forming a second mold in the first mold wherein the second mold comprises a negative prism array pattern. The retroreflective sheeting is then formed in the second mold and removed therefrom.
The invention has many advantages including providing a broader more uniform retroreflection profile than a single or double orientation cube corner prism sheeting. The invention is useful for truck conspicuity enhancement marking.